ABSTRACT
The di-pi-methane rearrangement is one of those particularly general organic photochemical reactions with fascinating mechanistic complexity in conjunction with remarkable consistency and beauty. The requirement for this photochemical reaction is a reactant with π groups bonded to a single carbon atom. The most common variations lead a reactant divinylmethane to a vinylcyclopropane or an arylvinylmethane reactant to an arylcyclopropane. One of the striking facets of the di-pi-methane rearrangement is its dependence on reaction multiplicity. Originally, it was felt that acyclic di-pi-methane systems were capable of rearrangement only via the singlet excited state, and for the most part this is still a valid view. In summary regarding di-pi-methane reversibility, it may be stated that the reaction directionality is determined both by structural features and also by electronic factors including multiplicity. The di-pi-methane rearrangement is particularly susceptible to control of its regioselectivity by substituents.
